1. Field of the Invention
The present invention relates to a semiconductor device and a production method thereof and, more particularly to a semiconductor device having a bump electrode and a production method thereof.
2. Description of the Related Art
FIG. 23 is a cross-sectional view of a conventional semiconductor device. As shown in this figure, the semiconductor device comprises a semiconductor chip 1 including a silicon substrate. An electronic circuit is formed on one surface of the silicon substrate. Bumps 1a that serve as electrodes are formed in a peripheral area around the electronic circuit portion. The semiconductor chip 1 is attached to a die pad 2. The bumps 1a are electrically connected to inner leads 4a of a lead frame 4 via metal wires 3. The semiconductor chip 1, the die pad 2, metal wires 3, and the inner leads 4a of the lead frame 4 are encapsulated with a molding resin 5.
A semiconductor device having such a configuration described above may be produced as follows. First, a semiconductor chip is attached to a die pad 2 with a metal-based solder or a resin-based adhesive. Wire bonding is then performed so as to electrically connect bumps 1a to inner leads 4a of a lead frame 4 via metal wires 3 such as gold wires. These elements are encapsulated with a molding resin 5 such as an epoxy resin, and thus a complete semiconductor device is obtained.
The size of conventional semiconductor device described above, however, is considerably greater than that of the semiconductor chip 1, since the semiconductor chip 1 attached on the die pad 2 is encapsulated with the molding resin 5 together with the inner leads 4a connected to the semiconductor chip 1 via the metal wires 3. Therefore, to mount semiconductor devices of this type on a circuit board, a large area on the circuit board is required for each semiconductor device. This means that it is difficult to achieve high density mounting of semiconductor devices, and that it is impossible to meet the increasing requirement of miniaturization in various electronic systems. Furthermore, in the conventional type semiconductor device mounted on a circuit board via a lead frame 4, heat cycles give rise to thermal stress between the lead frame 4 and the circuit board due to the difference in thermal expansion between the lead frame 4 and the circuit board, which may result in damage in the connecting portion between the lead frame 4 and the circuit board. Thus, the conventional semiconductor device has a problem with electrical reliability.
Wireless bonding is known as another technique to attach a semiconductor device on a lead frame. In this technique, the surface of a semiconductor chip on which an electronic circuit portion or the like is formed is directed down (in a face-down fashion), and electrodes are directly connected to a lead frame. In this technique, a semiconductor chip can be attached on a lead frame more easily as compared to the technique of wire bonding, and thus this technique is suitable for automated production.
FIG. 24 is a cross-sectional view of a semiconductor device according to the conventional wireless bonding technique. FIG. 25 is a perspective view of a semiconductor chip used in this semiconductor device, wherein the figure illustrates the semiconductor chip seen to its back side. In this type of semiconductor device, the face of a semiconductor chip 1 is directed down, and the semiconductor chip 1 is heated so as to connect bumps 1a to inner leads 4a of a lead frame 4 whereby the semiconductor chip 1 is electrically and mechanically connected to the lead frame 4, and held on it. The semiconductor chip 1 is then encapsulated with a molding resin 5 into an integral form so that an electric circuit portion formed on the semiconductor chip 1 and other elements such as connections between the semiconductor chip and the lead frame 4 are isolated from the outside environment.
This type of semiconductor device operates as follows. When an electric signal is applied from the outside to the outer lead 4b, the signal is transmitted to the semiconductor chip 1 via the inner lead 4a and the bump 1a. Similarly, an electric signal generated inside of the semiconductor chip 1 is output to the outside via the bump 1a and the inner and outer leads 4a, 4b of the lead frame 4. During the operation, the electric circuit of the semiconductor chip 1 generates heat, which is radiated to the outside via the molding resin 5 and the lead frame 4.
In the conventional type of semiconductor device described above, however, since the semiconductor chip 1 is connected to the lead frame 4 only via the bumps 1a, only a small amount of heat can be radiated via the lead frame 4, and the heat removal from the semiconductor chip 1 to the outside is mainly via the molding resin with thermal conductivity less than that of the lead frame 4. As a result, there is a problem that this type of semiconductor device is poor in heat removal from the semiconductor chip 1 to the outside, which may result in an increase in temperature beyond the allowable maximum limit. Thus, this type of semiconductor device has problems regarding the reliability or the maintenance of its functions.
The conventional technique to produce the semiconductor device will be described in more detail below.
FIG. 26 is a cross-sectional view illustrating a production method of the conventional semiconductor device. As shown in FIG. 26, a semiconductor chip 1 having electrodes 6 and bumps 1a formed on the electrodes 6 (refer to FIG. 27) is placed into a cavity between an upper die 7 and a lower die 8, and then clamped by the upper and lower dies 7 and 8. The molding resin 5 is injected into the cavity and cured so as to encapsulate the semiconductor chip 1. The height H.sub.1 of the cavity (refer to FIG. 26) is equal to the height H.sub.2 of the semiconductor chip 1 as measured from the bottom to the top of the bump 1a (refer to FIG. 27). Theoretically, there is no gap between the bumps 1a and the upper die 7. In practice, however, there is often a gap g (refer to FIG. 26) arising from the variations in the size of the lower die 8 and the height of the bumps 1a. If there is a gap g, the molding resin 5 intrudes into the gap g when the molding resin 5 is injected. As a result, thin burrs 9 of the molding resin 5 are formed on the bumps 1a.
In the conventional method of producing a semiconductor device, as described above, since the height H.sub.1 of the cavity formed between the upper and lower dies 7 and 8 is equal to the height H.sub.2 of the bump 1a, errors in these heights lead to the formation of the thin burrs 9 of the molding resin 5 on the bumps 1a.
FIG. 29 is a cross-sectional view of another semiconductor device according to a conventional technique. The semiconductor device comprises only a semiconductor chip 1 which has a number of protruding bumps 1a on its upper surface, without encapsulation in the molding resin 5. In this type of semiconductor device, the semiconductor chip 1 is exposed to the outside without protection from the outside environment.
The semiconductor device of this type has a problem in that there is a possibility that the semiconductor chip 1 may be damaged because there is no encapsulation by the molding resin 5 for protecting the semiconductor chip 1.